The invention pertains to photo detectors, and particularly to ultraviolet (UV) light photo detectors. More particularly, the invention pertains to back-illuminated UV photo detectors.
In the art, there have been several GaN-based p-n and p-i-n junction UV photodiodes. Here, the photovoltaic (zero bias) responsivities have been typically in the range of 0.1-0.12 ampere per watt (A/W), corresponding to external quantum efficiencies (i.e., collected electron-hole pairs per photon) of 30 to 40 percent. Under reverse bias, there has been 0.15 A/W or a 50-percent quantum efficiency at 364 nanometers (nm). These results indicate that even excluding the reflection loss on the GaN surface (minus 20 percent), there is still a considerable internal loss in these detectors. One common feature of these photodiodes is that the light enters into the detector through a 0.2 to 0.3 micron top p-GaN layer, in which most of the absorption and photogeneration take place near the p-GaN surface. This renders a diffusion-limited photocurrent and possibly substantial carrier loss due to surface recombination. The situation worsens at shorter wavelengths due to stronger absorption near the surface. Such homojunction photodiodes have been modeled and estimated to have a maximum responsivity of 0.13 A/W at 362 nm, or a quantum efficiency of 45 percent at zero bias. Under reverse bias, the responsivity is expected to be higher due to field assisted diffusion, but this generally leads to leakage current and associated shot noises.
Improvement in efficiency can be obtained by reducing the p-GaN top layer thickness. But overly thinning the p-GaN layer results in large RC delay due to the poor conductivity of the p-GaN. On the other hand, using p-AlGaN as the top electrode results in junction degradation due to the lower quality of p-AlGaN. This represents a bigger challenge for higher Al mole fractions. Schottky type photodiodes have shown higher responsivity because the carriers are generated within the junction and are collected rather efficiently. A responsivity of about 0.18 A/W on n-GaN Schottky photodiodes under a minus five-volt bias, has been noted. However, the leakage current in these diodes was several orders of magnitude higher than p-n or p-i-n junction type photodiodes. The Schottky photodiodes also have the drawback of light absorption by the metal contact.
The present invention is a photo detector or photodiode that provides high efficiency in the detection of UV light. It is an aluminum gallium nitride (AlGaN) based ultra-violet photodiode that has application in flame sensing and other ultraviolet detection uses. Light comes into the detector from the substrate side, contrary to typical detectors. In other words, it is a back-illuminated GaN/AlGaN ultraviolet (UV) heterojunction photodiode with a high quantum efficiency. A photovoltaic (zero bias) responsivity (which correlates to a quantum efficiency) of 0.2 A/W at 355 nm was achieved. This responsivity or efficiency is higher than all previously reported AlGaN and SiC based photodiodes. The present device can be operated under zero bias (photovoltaic mode) or reverse bias depending on the specific applications. The improved efficiency primarily arises from the use of an AlGaN/GaN heterojunction in which photons are absorbed within the p-n junction thus eliminates carrier losses due to surface recombination and diffusion processes in previously reported homojunction devices. Dark impedance and a visible rejection ratio much higher than typical photo conductors or detectors were obtained.
The heterojunction UV detection diode is based on a p-i-n structure. The structure consists of four AlGaN layers deposited on sapphire substrate. The layers include an AlN buffer layer, an n-type AlxGa(1xe2x88x92x)N layer, an undoped AlyGa(1xe2x88x92y)N absorption layer and a p-type AlzGa(1xe2x88x92z)N layer. The undoped i-type AlyGa1xe2x88x92yN absorption layer is interposed between the p-type AlzGa1xe2x88x92zN layer and the n-type AlxGa1xe2x88x92xN layer. The bandpass wavelength selectivity can be adjusted by varying the Al mole fractions in the n-type AlxGa1xe2x88x92xN layer and the undoped AlyGa1xe2x88x92yN absorption layer. The ranges of the mole fractions are: 0 less than x less than 0.5, 0xe2x89xa6y less than 0.5, 0xe2x89xa6z less than 0.5, x greater than y, and zxe2x89xa7y. In this configuration, the spectral range of detection is determined by the absorption edges of the n-type AlxGa(1xe2x88x92x)N layer and the undoped AlyGa(1xe2x88x92y)N absorption layer. For example, when x=0.4 and y=z=0 are used, the detector is sensitive in the 275 nm to 365 nm spectral range, suitable for flame sensing in most boilers and turbines. The UV light in the detection range enters from the sapphire side and essentially is absorbed only by the undoped AlyGa(1xe2x88x92y)N layer. This ensures that most photo-generated carriers are within the p-n junction region and are collected efficiently.
In sum, a GaN/AlGaN UV photodiode with a very high quantum efficiency is disclosed, with a photovoltaic (zero bias) responsivity of at least 0.2 A/W at a 355-nm wavelength. This improvement is primarily attributed to the use of an AlGaN/GaN heterojunction in which photons are absorbed within the p-n junction and away from the surface and thus carrier loss due to surface recombination and diffusion processes are eliminated. Very high dark impedance and a large visible rejection ratio can be obtained. These results indicate a high quality GaN/AlGaN interface and efficient photocarrier collection in the photodiode and represent a significant improvement over related art GaN-based homojunction photodiodes.
Prior to this invention, AlGaN and SiC based solid state UV detectors included photoconductors and various photodiodes of both Schottky and p-n junction types. Each of these photoconductors has simplicity but is known to have a very slow response due to persistent photoconductivity (PPC). It also exhibits substantial dark current which drifts in a large magnitude over required operating temperature ranges (e.g. from minus 40 to plus 120 degrees C.), rendering it unsuitable for DC mode detection. In addition, the photoconductive characteristics are difficult to reproduce due to its reliance on defect related effects in AlGaN, which imposes further limitations to such photoconductor as a viable product. The present photodiode does not have PPC and drift problems, and can be used in both DC and AC detection modes. It also has excellent reproducibility.
In contrast to AlGaN based Schottky photodiodes, the present photodiode provides much lower leakage current and smaller temperature dependence, and thus is more suitable for applications requiring wide operating temperature ranges and high sensitivities.
In comparison to conventional AlGaN and SiC based p-n junction photodiodes, the present photodiode provides higher responsivity due to the use of heterojunctions and back illumination. In related art AlGaN and SiC homojunction devices, carrier losses arise from surface recombination and diffusion process because most carriers are generated near the surface. In the present device, the photons are absorbed within the p-n junction and therefore the photo-generated carriers are collected efficiently. In addition, the sapphire-air and sapphire-AlGaN interfaces give a total reflection of about 12 percent, compared with about 20 percent reflection directly on AlGaN surface.
The improvement over SiC based devices also includes a sharper long wavelength cutoff due to the direct band gap of AlGaN, and a tunable band-pass wavelength selectivity, which can be achieved by adjusting the Al mole fractions in the n-type AlxGa(1xe2x88x92x)N layer and the undoped AlyGa(1xe2x88x92y)N layer. Thus, the present device can be fabricated for the detection of a specific narrow band of UV wavelengths without using external filters.